Optical transmitter and optical transmission method

Abstract

It is difficult to carry out automatic bias control to an optical modulator without a dither signal with a simple configuration, therefore, an optical transmitter comprising the controller configured to adjust an adjusted bias voltage based on the coupled optical signal, the adjusted bias voltage being one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator and a third bias voltage applied to the third optical modulator and maintain a value of bias voltages other than the adjusted voltage in the first bias voltage, the second bias voltage and the third voltage.

What is claimed is: 
     
         1 . An optical transmitter comprising:
 a light source configured to output light;   a splitter configured to split the light to a first branched light and a second branched light;   a first optical modulator configured to modulate the first branched light and output a first modulated optical signal;   a second optical modulator configured to modulate the second branched light and output a second modulated optical signal;   a third optical modulator configured to adjust a phase difference between the first modulated optical signal and the second modulated optical signal;   a coupler configured to couple the first modulated optical signal and the second modulated optical signal and output a coupled optical signal; and   a controller configured to adjust an adjusted bias voltage based on the coupled optical signal, the adjusted bias voltage being one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator and a third bias voltage applied to the third optical modulator,   wherein the controller is configured to:
 maintain a value of bias voltages other than the adjusted voltage in the first bias voltage, the second bias voltage and the third bias voltage; 
 measure an error of the coupled optical signal by using a modulation error ratio (MER); 
 adjust the adjusted voltage when the error exceeds a threshold; and 
 maintain a value of the adjusted bias voltage when the error of the coupled optical signal does not exceed the threshold while adjusting for the adjusted bias voltage. 
   
     
     
         2 . The optical transmitter according to  claim 1 , wherein the controller is configured to increase and decrease the value of the adjusted bias voltage by an amount of change. 
     
     
         3 . The optical transmitter according to  claim 2 , wherein the controller is configured to calculate the amount of change based on one of the first bias voltage, the second bias voltage and the third bias voltage. 
     
     
         4 . The optical transmitter according to  claim 2 , wherein the controller is configured to calculate the amount of change based on temperature of the optical transmitter. 
     
     
         5 . The optical transmitter according to  claim 2 , wherein the controller is configured to calculate the amount of change based on a value of a root mean square (RMS) of a driving signal for modulating the first branched light or a value of RMS of a driving signal for modulating the second branched light. 
     
     
         6 . The optical transmitter according to  claim 2 , wherein the controller is configured to calculate the amount of change based on a used time of the optical transmitter. 
     
     
         7 . The optical transmitter according to  claim 3 , wherein the controller is configured to calculate the amount of change based on temperature of the optical transmitter. 
     
     
         8 . The optical transmitter according to  claim 3 , wherein the controller is configured to calculate the amount of change based on a value of root mean square (RMS) of a driving signal for modulating the first branched light or a value of RMS of a driving signal for modulating the second branched light. 
     
     
         9 . The optical transmitter according to  claim 3 , wherein the controller is configured to calculate the amount of change based on a used time of the optical transmitter. 
     
     
         10 . The optical transmitter according to  claim 4 , wherein the controller is configured to calculate the amount of change based on a value of root mean square (RMS) of a driving signal for modulating the first branched light or a value of RMS of a driving signal for modulating the second branched light. 
     
     
         11 . The optical transmitter according to  claim 4 , wherein the controller is configured to calculate the amount of change based on a used time of the optical transmitter. 
     
     
         12 . The optical transmitter according to  claim 5 , wherein the controller is configured to calculate the amount of change based on a used time of the optical transmitter. 
     
     
         13 . An optical transmission method comprising:
 outputting light;   splitting the light to a first branched light and a second branched light;   modulating the first branched light and outputting a first modulated optical signal by a first optical modulator;   modulating the second branched light and outputting a second modulated optical signal by a second optical modulator;   adjusting a phase difference between the first modulated optical signal and the second modulated optical signal by a third optical modulator;   coupling the first modulated optical signal and the second modulated optical signal and outputting a coupled optical signal;   adjusting an adjusted bias voltage based on the coupled optical signal, the adjusted bias voltage being one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator and a third bias voltage applied to the third optical modulator;   maintaining a value of bias voltages other than the adjusted voltage in the first bias voltage, the second bias voltage and the third bias voltage;   measuring an error of the coupled optical signal by using a modulation error ratio (MER);   adjusting the adjusted voltage when the error exceeds a threshold; and   maintaining a value of the adjusted bias voltage when the error of the coupled optical signal does not exceed the threshold while adjusting for the adjusted bias voltage.